This invention relates to a magnetic tape having an abrasive and cleaning effect for those parts of the magnetic tape cassette and the recording/play-back unit which come into contact with the magnetic layer, consisting of a non-magnetic layer support and a dispersion layer applied thereto which contains hard magnetic pigments and non-magnetic pigments having a particle size of less than 2 .mu.m and also polymeric binders and, optionally, other additives, such as dispersants, hardeners, plasticizers and lubricants.
When a magnetic recording medium, for example magnetic tape, is used over a prolonged period, abrasion occurs and the resulting dust builds up on the surface of the magnetic head. Dust from the environment can also build up on, and hence soils, the magnetic head. Deposits of this kind are one cause of reduced audio and video quality which leads inter alia to a fall in level during recording and playback. Another cause for the fall in level is so-called head degradation. This is attributable to the conversion of the substantially monocrystalline magnetic head surface into a polycrystalline state under the stress effect of the magnetic tape drawn over the magnetic head.
A general development objective in the production of magnetic tapes is to keep the percentage content of non-magnetic material in the storage layer as small as possible in order not to impair the magnetic properties.
However, where magnetic iron oxides--possibly doped with cobalt--are used as the storage particles, there are two reasons for using non-magnetic pigments in addition to the polymeric binder which is necessary for binding the magnetic pigments and for the mechanical running behavior of the magnetic tape, namely:
in many cases, the abrasion resistance of the magnetic layer is only made satisfactory by the addition of such non-magnetic pigments PA1 where the magnetic tape is used for video recording, the Mohs' hardness of the magnetic iron oxides is not sufficient to guarantee the necessary constant, but light polishing of the video magnetic heads made of ferrite. The addition of hard abrasives to the layer formulation makes it possible to establish a desirable minimal and uniform head abrasion which ensures good magnetic contact between tape and head. Corresponding formulations are known from U.S. Pat. Nos. 3,630,910, 4,379,800 and DE 39 10 488.
Where magnetic pigments of CrO.sub.2 are used, the opposite situation arises, i.e. the heads are in danger of excessive abrasion by the magnetic pigment as a result of the considerable Mohs' hardness of that pigment. Attempts have been made to counteract this danger by using relatively soft inorganic pigments, such as ZnO, and/or by improved coating of the magnetic pigments with organic material, as known for example from Transactions on Magnetics, Vol.-Mag. 23, No. 1, January 1987, pages 103-105. Surface smoothing of the magnetic tape by calendering can also have a controlling effect in establishing the desired minimal head abrasion. From this point of view, the use of hard non-magnetic pigments is problematical where CrO.sub.2 is used. DE 25 54 146 describes a process for cleaning the magnetic heads of a video recorder with a video monitor, the magnetic pigment being CrO.sub.2 to which Al.sub.2 O.sub.3, which is known to have a Mohs' value of 9, is added as non-magnetic pigment. A video test image is recorded on a correspondingly produced magnetic tape and the tape is loaded into the recorder to be cleaned and played until the test image is adequately reproduced. At this point, the recorder has to be stopped immediately and the test cassette removed. It is clear that this process is extremely critical and, unless carried out properly, leads to scratching of the video heads. This magnetic tape has an Ra value of 0.05 to 0.12 .mu.m, i.e. is far too rough for use as a video tape.
In addition, U.S. Pat. No. 4,138,229 and U.S. Pat. No. 4,397,911 describe cleaning tapes for magnetic recording heads which do not contain a magnetic pigment, but rather inorganic pigments differing in hardness and particle diameter or mixtures of inorganic and organic polymer powders
Irrespective of this, it has been found that a certain polishing effect is desirable not only for the video head. Partial abrasion is also useful for other parts coming into contact with the magnetic layer, above all the video cassette, because roughness peaks of such parts, for example tape guide elements or tape deflecting elements, can damage the touching magnetic layer, lead to visible and audible interference during play-back.
However, investigations have shown that roughness peaks of the type in question cannot be eliminated by the magnetic pigments because these pigments, which are used to store the signals, are made up of very fine particles to minimize signal interference and to ensure a high recording density. Although, given adequate hardness, fine-particle magnetic pigments of the type in question can have a light polishing effect on a part of the cassette coming into contact with the magnetic layer, troublesome and damaging roughness peaks are only significantly smoothed after relatively long running of the tape, so that the tape of a video cassette may possibly be scratched over its entire length, leading to malfunctions because, as already mentioned, longitudinal scratches--depending on their depth--can appear on the screen as long-lasting, visible and troublesome cross-streaks.
Although, as expected, this laborious light polishing is carried out more quickly by the relatively hard CrO.sub.2 pigments than by the softer iron oxide particles, the process is nevertheless far too slow with both types of pigment, so that it is not possible to avoid damage to long sections of tape during transport in the video cassette. Accordingly, the pigments according to the prior art may only be used in conjunction with thoroughly premachined, smooth cassette components with which the magnetic tape comes into contact, resulting in a corresponding increase in the manufacturing costs of a video cassette.